1. Field of the Invention
The present invention relates to a method and apparatus for sterilizing water (water to be treated) (hereinafter, “water to be treated” will be referred to as “for-treatment water”) such as water used for drinking and cooking or waste water discharged from hydroponics and to a hydroponics system using the apparatus.
2. Description of the Related Art
Heretofore, for example, to remove microorganisms such as bacteria, fungi and protozoans contained in water intended for drinking and cooking such as tap water reserved in a reservoir, a method has been employed in which a porous filter material capable of collecting these microorganisms is placed in a water channel so as to make the microorganisms stick to the filter material and thereby purify the water.
Further, in addition to this, there is also a method in which chlorine and ozone are produced by electrolysis caused by electrodes immersed in water intended for drinking and cooking and microorganisms contained in the water are eliminated by sterilizing effects of the produced chlorine and ozone.
Meanwhile, in hydroponics, a solution prepared by dissolving a fertilizer in water in an amount suitable for cultivation of crops is circulated through a cultivation bed so as to suitably grow crops planted in the cultivation bed. However, the inside of the cultivation bed through which the fertilizer solution is circulated is an environment suitable for proliferation of pathogenic bacteria such as Fusarium (a type of fungi), and if the Fusariums are proliferated in the cultivation bed, they damage roots of the crops and cause the crops to die. Accordingly, in this case as well, a means for killing or decreasing pathogenic bacteria in the fertilizer solution is needed.
Meanwhile, such a filter material as described above collects microorganisms by receiving them in fine pores formed on its surface. Therefore, unless the microorganisms collide with or pass by the surface of the filter material, they cannot be stuck to the filter material. Thus, the effect of causing the microorganisms to stick to the filter material is limited.
Microorganisms are generally negatively charged (reference: “Basic and Application Technique of Electrolyte” authored by Masaki Matsuo, published by Gihodo Syuppan. For reference purposes, some microorganisms are positively charged.) Thus, when the filter material is composed of, for example, carbon fibers which are conductive materials and a positive potential is applied to the carbon fibers, microorganisms can be attracted to the carbon fibers so as to be collected thereby.
However, when a positive potential is applied to the carbon fibers, oxygen is produced on the surfaces of the carbon fibers by electrolysis, so that trace amounts of nutritional elements contained in for-treatment water such as manganese (Mn) and iron (Fe) are oxidized and precipitated and removed as manganese oxide, i.e., MnO, Mn2O3, MnO2 or Mn2O7, and iron oxide, i.e., FeO or Fe2O3. Thus, particularly, when a fertilizer solution to be used in hydroponics is sterilized, the concentration of nutrients (minerals) contained in the fertilizer solution is reduced, thereby causing a problem that growth of plants is inhibited.